1. Field of the Invention
This invention is directed generally toward methods to electrochemically deposit semiconductors and for the electrochemical formation of epitaxial, thin-film, single-crystalline compound semiconductors. More specifically, this invention is directed toward a method for the electrochemical deposition of semiconductors by alternately depositing atomic layers of the component elements.
2. Prior Art
Presently, drawbacks to standard electrodeposition methods include the tendency to form amorphous or polycrystalline deposits, sometimes described as "cauliflower" deposits because of their convoluted morphology. The extensive grain boundary networks in polycrystalline materials increase their resistivity and provide recombination centers, and are thus detrimental to most applications. Improving the crystallinity of electrodeposited compound semiconductors is the next step in the evolution of electrodeposition as an important production technique for these compounds.
One example of the state of the art in the use of electrodeposition of elements to produce a compound semiconductor is disclosed in U.S. Pat. No. 4,581,108 issued to Kapur et al. Kapur teaches the alternated electrodeposition of elements, and the subsequent heating of these elements to form an essentially homogeneous layer of semiconductive compound. As is readily apparent, Kapur deposits relatively thick layers of the first element, typically 500 to 1000 atomic layers, before changing over and depositing 500 to 1000 atomic layers of the second element. Additionally, the actual formation of the semiconductive material in Kapur is achieved through the subsequent annealing of the layers to 400.degree. C., allowing the elements to interdiffuse and combine. Until the annealing process, no compound formation takes place.
The disadvantages of this prior art electrodeposition process, such as that disclosed in Kapur, involve both the electrodeposition of bulk phases of the individual elements and the need for a subsequent anneal after the electrodepostion. The anneal is required for the combination of the elements to form the compound. Difficulties can arise in obtaining an ordered single-crystalline deposit homogeneously throughout the whole deposit, since compound formation relies on the interdiffusion of the two elements. The annealing step is especially disadvantageous if a heterojunction device, is fabricated with the art, since diffusion across junctions increases with temperature.
Another example of the state of the art in the electrodeposition of compound semiconductors is disclosed in U.S. Pat. No. 4,400,244 issued to Kroger et al. Kroger teaches that compound semiconductors can be electrodeposited through codeposition. In other words, by combining both elements in a single solution and selecting an optimum potential for deposition, a stoichiometric deposit can be achieved. This method utilizes the underpotential deposition (UPD) of one of the elements. UPD is the energy advantage afforded by formation of a compound on the surface. In order to limit the deposit composition to nearly a 1:1 stoichiometry, the potential for deposition is selected so that bulk Cd is not formed, and that Cd will only deposit onto previously deposited Te. The concentration of TeO.sub.2 is kept much lower than that of Cd.sup.+2 so that deposition of Te is the limiting step. All deposited Te is quantitatively reacted with the excess of Cd.sup.+2 ion.
The disadvantages of this prior art electrodepostion process lie in the resulting deposit structure. Although the stoichiometry is satisfactory, the structure of the deposits is usually polycrystalline, frequently having very convoluted morphologies. This generally results due to a lack of control over the rates of nucleation and growth of the deposits. Three-dimensional nuclei are formed and result in three-dimensional growth and polycrystallinity. Epitaxial electrodeposition is desired and necessitates suppression of these nuclei.
There are a wide variety of applications for compound semiconductors. For example, they are used in photovoltaics, luminescent displays, radiation detectors, lasers and laser windows, infrared detectors, and Vidicon imaging devices. Compound semiconductors are the central components in emerging electro-optical technologies based on layered structures. They display a variety of band gaps and some form solid solutions. The ability to control stoichiometry in these solid solutions results in the ability to vary the band gap as a function of its composition. The mercury-cadmium-telluride (MCT) system is an important example of band gap engineering.
Electrodeposition of semiconductors is a potential low cost, room temperature production technique. All previous work in compound semiconductor electrodeposition has resulted in polycrystalline deposits. There are four primary reasons for the formation of polycrystalline deposits: three-dimensional nuclei formation; the absence of an ordered substrate structure; the absence of a lattice match between substrate and deposit; and substrate, solvent, reactant and electrolyte contamination. To date, these problems have not been overcome.
Vacuum-based methods for compound semiconductor growth (e.g., molecular beam epitaxy (MBE) or vapor phase epitaxy (VPE)) involve some of the same problems encountered in electrodeposition, such as the need for careful control of reactant fluxes in order to obtain epitaxial deposits. Atomic layer epitaxy (ALE) represents a group of techniques currently under development which allow less stringent control of such growth parameters. An example of the state of the art of atomic layer epitaxy is disclosed in U.S. Pat. No. 4,058,430 issued to Suntola et al. Unique to ALE is compound growth one atomic layer at a time. These techniques rely on surface-specific, self-limiting reactions, which result in only an atomic layer of reactivity. If the reactant is an elemental vapor, the substrate temperature is adjusted so that bulk deposits sublime while the first atomic layer remains due to an enhanced stability resulting from compound formation. After evacuation of the first element, a similar procedure is performed with the second element. For a compound such as CdTe, an atomic layer of Cd is formed followed by an atomic layer of Te. Thin film growth is achieved by repeating this cycle.